This invention relates to discharge lamp operating circuits, and more particularly, to high frequency circuits for efficiently operating low wattage, high intensity discharge (HID) lamps.
Conventional ballast circuits for operating HID lamps have been constructed from leakage reactance transformers and may or may not include a spike or pulse starter. Typically, the discharge current through the lamp is controlled by the inductive reactance of the transformer core at a 60 Hz line frequency. As will be discussed hereinafter, such ballasts are not particularly suitable for the much smaller size, lower wattage high pressure metal halide arc discharge lamps currently being introduced. For example, one such lamp is discussed in U.S. Pat. No. 4,161,672, which also describes the use of double-ended arc tubes for such lamps. Further, a copending application U.S. Ser. No. 132,933, filed Mar. 24, 1980 and assigned to the present assignee, describes a low wattage metal halide arc discharge lamp having a press-sealed single-ended arc tube, that is to say, an arc tube in which both electrodes are located in a press-seal at one end of the arc tube. Practical designs of such lamps have ranged from 100 watts to less than 10 watts.
A ballast intended for such low power arc discharge lamps, particularly of the metal halide group, must satisfy at least the following requirements:
(a) produce glow-to-arc conversion across the electrodes; PA1 (b) maintain sufficient open circuit voltage to overcome any lamp voltage reignition during warm-up; and PA1 (c) control lamp power efficiently.
Ballast circuits designed to operate HID lamps at the line frequency (e.g., 60 Hz) generally satisfy the first requirement by producing output open circuit voltages of at least a factor of two above the input line voltage. Unless the open circuit voltage approaches 500-600 volts, however, a series igniter pulse of several kilovolts is generally employed to promote the initial interelectrode glow breakdown. In addition, the need for high open circuit voltage ballast circuits is greatest for the metal halide lamps where reignition voltages can be several hundred volts. As a consequence of high open circuit voltage, the ballast circuit is usually only 50-60% efficient at converting the electrical input power into the discharge.
An unfortunate characteristic of the aforementioned miniature, low-wattage metal halide lamps is the very rapid plasma deionization to which they are subject. For example, in operation on 60 Hz alternating current, deionization may nearly eliminate charge carrying species between half cycles so that a very high restriking voltage is required to be provided by the ballast. In view of these deionization limitations associated with low frequency operation of miniature metal halide HID lamps, the use of conventional 60 Hz ballasts has many disadvantages.
Accordingly, in the interests of a compact and efficient design, high frequency ballast operation of such lamps has been described, such as, for example, in U.S. Pat. No. 4,170,746. Our invention is particularly concerned with providing further improvements in overall system efficiency in addition to other advantages.